Such phosphors are disclosed in U.S. Pat. Nos. 4,225,653 and 4,387,141. The phosphor composition in both patents has the formula YNb.sub.x Ta.sub.1-x O.sub.4, where x is 0 to about 0.15. A mixture of raw materials is fired and then blended with a flux, which can be lithium sulfate, lithium chloride, or a barium chloride-lithium chloride eutectic mix, before refiring. The weight of the flux ranges from about 20 to about 50 weight percent of the mixture.